ATM S 111, Global Warming:
Understanding the Forecast
DARGAN M. W. FRIERSON
DEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 5: 10/15/2015
Floods and droughts
Floods and droughts: two sides of a catastrophic coin
A wetter world (but not everywhere)
Are floods increasing?
Defining drought
What do the oceans have to do with drought?
The drying of southern Australia
The plough and its followers: farming and rainfall
Rainiest and Driest Places on Earth
 These are just places that have weather stations
 There are likely more extreme places that aren’t being
measured
Rainiest spot on Earth:
Mount Waialeale, Hawaii (pictured): 460 inches/year
(11.7 meters)
Rainiest spot may actually be Lloro, Colombia: 523.6
inches/year (13.3 meters – this is just an estimated
amount though)
Rainiest year on record:
905 inches (almost 23 meters) in Cherrapunji, India
In the foothills of the Himalayas
Rainiest day ever:
Reunion Island (500 miles off coast of Madagascar)
72 inches (1.86 m) in 24 hours!!!
Also just five years ago, Reunion Island set the record
for 72 hour rainfall: 155 inches (3.9 m) during a tropical
cyclone
Extreme Precipitation
 All of the above examples are associated with
mountains

More rain on windward side of mountains


When air is lifted up the slope
These extremes are localized rainy areas

Just like the Olympic Peninsula here in Washington!
What’s Required for Rain?
 To get rain, you need water vapor and rising
motion

Condensation (water vapor turning to liquid water) happens
when moist air cools

And this cooling almost always happens with rising motion
 Let’s take a look at precipitation on a global scale…
Outline
 Precipitation around the world
 Tropical rainy regions and subtropical deserts
 Monsoons
 Storm tracks
 Changes that are expected in the future
 Wet areas get wetter, dry regions persist/expand
 More heavy rain events
 Changes that have been observed
 Increased rainfall/floods?
 Droughts in Australia
 Droughts in the Sahel
Average Precipitation
 Precipitation (mm/day):
Rainiest spots on global scale are narrow bands near the equator
Many of the driest places on Earth are over the ocean!
Most deserts are around 30 degrees latitude
“Hadley Circulation”
Air rises above the warmest ocean surface
(hot air rises)
Sinks around 30
degrees latitude
(where deserts are)
Michelle Weirathmueller
Seasonal March of Tropical Rains
 Rain shifts northward in Northern summer (JJA),
southward in Southern summer (DJF)
“Convergence zone”: where winds come together
“Inter-tropical convergence zone” (ITCZ): the tropical band.
The ITCZ follows the warmest ocean temperatures as they shift with the seasons
Rainfall and Vegetation
 Vegetation follows the seasonal march of rainfall
Monsoon Circulations
 Land heats up in the summer, leads to rising/rain
India, Africa, Australia, etc
all experience strong
monsoons
colder
hotter
El Nino cycles greatly
modify the
location/intensity of
monsoons in IndiaIn&
Australia
Midlatitude Precipitation
 Midlatitude precipitation is associated with storm
systems


Rising motion is generated in particular regions of the storms
Take our other classes to learn more! (like ATM S 101)
“Storm Tracks”
 Precipitation (mm/day):
Midlatitude precipitation is in storm tracks: preferred locations for storms
Seasonal March of Midlatitude Rains
 Rains shift and change in intensity in midlatitudes
seasonally too
Notice how close together rainy and dry areas are, in tropics and midlatitudes….
Small shifts in precipitation can cause a massive change locally!
Whether rainy regions shift is a key uncertainty for predicting changes with warming
January
Floods
and droughts: two sides of a
ST
ST
catastrophic
coinwetter while dry regions become
Wet regions become
ITCZ
drier
M
ITCZ
M
SPCZ
M
SACZ
ST
M=monsoon, CZ=convergence zone, ST=storm track:
Climate models show these features getting stronger with
July
warming.
ST
ST
M
ITCZ
SPCZ
ST
M
ITCZ
M
 Climate model
projections of
precipitation change
Wet gets wetter,
dry regions expand
Stippling: where
models agree
January
July
Why?
As the atmosphere
warms, water vapor
concentrations
increase
Winds bring in even
more moisture into
the rainy regions
W = wind from west
T = wind from NE
blue = rainbelts
Global Warming Rain Responses
 Wet get wetter
 More water vapor is brought into the regions that are
already rainy
 Specifically, tropical regions, monsoons, storm tracks,
and high latitudes are expected to get rainier
Global Warming Rain Responses
 Dry regions dry/expand
 Many subtropical regions (in between tropics &
midlatitudes) are expected to dry
More vapor taken out of dry regions
 More evaporation from dry land surfaces at higher temperatures
 Tendency for midlatitude weather to shift poleward and Hadley
cell to widen

Increased evaporation
 A warmer atmosphere is like a bigger sponge
 Sucks up more moisture from the land
 Simply because warmer air holds more moisture
Leads to drier soil
in most locations
Soil moisture
change by 2100 
Hadley Cell Widening
 The Hadley circulation has widened, and this is expected
to continue
 Dry regions expand poleward when the Had. cell widens
I was a consultant about
Hadley cell expansion for Al
Gore’s most recent book The Future
Global Warming Rain Responses: Caveat
 There’s a lot of uncertainty about specific
precipitation responses though

Regional responses could change significantly due to shifts in
rising motion


Small shifts can make a big difference for rainfall!
Precipitation is much harder to predict than temperature
Some Additional Predictions
 Southwestern North America predicted to dry
dramatically

As bad as the Dust Bowl by 2060 in some models
Dust Bowl drought level
Seager et al 2007
US Predictions
 US predictions
How about the most intense storms?
The strongest downpours require a lot of water vapor in the
atmosphere
Warmer temperatures  more water vapor
Thus, heavy rainfall events should become more extreme
Also heavy snowfall events!
Are heavy rain events increasing?
Very heavy events have been
increasing
And heavy precipitation events in
the US are projected to get worse
Are heavy rain events increasing?
The global picture on how much heavy rain is increasing
Global increase in heavy rain days
Locations where heavy
rain is changing
disproportionally
(i.e., not just due to changes
in average precipitation)
Let’s look at some recent
flood events…
Nashville, TN, May 2010
A 1000 year flood event (should happen once every 1000 yrs)
2 day rainfall: 13.57” at Nashville airport
Country Music Hall of Fame
LP Field (home of Titans)
Are floods increasing?
Extreme flood events should increase with increased heavy rain
Trends in the frequency of flood events are difficult to quantify
because
Rare events, especially when considered season-by-season.
it’s difficult to establish statistical significance when dealing with rare events.
Hourly rainfall data are available only over limited regions of the globe
River configurations and land use are continually changing
The first two are true for heat waves too
Do forests prevent flooding?
 A recent study says yes: Bradshaw et al., Global
Change Biol. 13, 1-17 (2007)


Analyzed broader scale data and found deforestation explained
14% of floods
They predict that decreasing forest area by 10% would increase
flood frequency 4-28% and duration 4-8%
 Less runoff, slower erosion when trees are present
Defining drought
Months or years with below normal water supply.
Usually from below average precipitation.
The definition is not quantitative.
Specific criteria (e.g., how long, how severe...) need to be specified.
Other factors such as population growth can create deficiencies in water
supply
(ie Lake Chad)
Shifts in Rain Collapsed the Mayan Civilization?
 The Mayan civilization was among the most vibrant
civilizations in Central America in 250-900 AD
 They abandoned their southern cities in the 8th & 9th
centuries
Southward shift of
rain band the main
cause?
The drying of southern Australia
Mean Australian Rainfall (mm)
1986 - 1995.
4000
2400
1800
1200
1000
800
600
500
400
300
200
150
100
0
250
Desert
The drying of southern Australia
Shift in midlatitude storm track southward is important for drying
The decline in rainfall in south-western Australia since the 1960s.
Source:http://www.ioci.org.au/publications/pdf/IOCI_Notes_Series2.pdf.
Drying effects on temperature
In most places (as in SE Australia, shown Note that in SW Australia, the drying
here) minimum temperatures are rising causes very high maximum temperature
rise too.
faster than the maximum (greenhouse
effect).
The drying of southern Australia
Other factors that may be playing a role
southward shift in the storm track due to the ozone hole
increased water demand due to rising temperatures
increased water demand due to population growth
Recent Droughts in the Middle East
 Severe droughts in the Middle East peaked around
2010

Four straight years (2006-2010) of drought in northern Syria
– worst drought in Syria’s recorded history
Precipitation deficit 1970-2010
Water deficit in July 2008
Links to Migration, Uprising?
 Drought displaced 1.5 million people in Syria by 2011
 75% of farmers experienced total crop failure
 80% of livestock was lost
 Many farmers forced to move to cities in extreme poverty
 A “stressor” for the revolution?
 Revolts started in March 2011
 Clearly many reasons for the revolution, climate is only one
 Other Arab Spring protests may have been
influenced by climate as well

Wheat prices were particularly high due to droughts (globally
and locally)
Recent Drought in Texas
 Multi-year drought, one of worst in Texas’ history
 2011 was worst year
 Ranchers forced to sell cattle, crops failed
 Caused higher food prices throughout the country
 Primarily caused by La Nina
 But severe La Nina impacts have become 20 times more likely
with global warming
Dust storm in Lubbock, TX
California Drought
 71% of the state is at least in “extreme drought”
 April 2015 snowpack only 5% of normal!
 Persistent ridge of high pressure has kept rain away while
much of US recovered
Folsom Reservoir in
July 2011 (97% capacity) &
January 2014 (17% capacity)
California Drought
 Lake Oroville 
 July 2011 and
August 2014
Sacramento and San Joaquin
Valleys, Jan ‘13 and ‘14
Three years of above average
rainfall needed for recovery
The Sahel region of Africa
the Sahel
SUDAN
Darfur
The Sahel is in between
desert and the region
drenched by the African
monsoon
The African Monsoon in full swing
Sahel drought
wet period
extended drought
The shift around 1970 is believed to be due to changing sea surface
temperature patterns in the tropical Atlantic (quite possibly driven by
aerosol forcing).
The disappearance of Lake Chad
Lake Chad
1972
1987
Some of My Group’s Research…
 Dr. Ting Hwang (finished her
Ph.D. last year)
 Studying shifts in tropical
precipitation in simulations of
20th century climate
Observed versus Modeled Precip Changes
Observed precipitation changes
for different latitude bands
Modeled precipitation changes
for different latitude bands
Models do a good job at simulating
this general drying…
Why the Shift?
 We have found (in previous work) that tropical
precipitation shifts towards the warmer hemisphere

Even if the warming happens far from the tropics!
 We find aerosol cooling causes
the southward shift in models


Aerosols are mostly in the Northern
Hemisphere
Rain shifts southward, away from the
Sahel
Pollution Caused Part of
the 1980s African
Drought?
• Our recent study suggests a possible
link of the 1980s drought with pollution
Shifts are Hard to Predict
 Shifts in rising motion or midlatitude storms could
happen due to:

Differences in forcings


Differences in feedbacks


E.g., aerosols cooling the oceans in places
E.g., cloud responses
Changes in ocean currents

A natural example that messes with rain patterns is El Niño
Summary of Floods and Droughts
 To get rain, you need water vapor and rising motion


Hot air rises: this causes rain in the tropics and in monsoons
(hot air rising over land in the summer)
Where we live (in midlatitudes), rain is associated with storm
systems
 With global warming we will have more water vapor
in the atmosphere



Warmer air can hold more moisture
Hence rainy places get rainier
Big downpours will be more intense
 Many land areas will dry out

More evaporation out of the land surfaces
Summary of Floods and Droughts
 Floods and droughts will doubtlessly be some of the
most important impacts of global warming



Hard to predict exactly though
Small shifts in location of rising motion can make a big
difference
Affected by many things
Forcings like aerosols
 Feedbacks like ice or cloud changes
 Natural variability like ocean current changes
